Compensation of refrigeration losses during the storage of liquefied, low-boiling gaseous mixtures

ABSTRACT

A process for the compensation of the refrigeration losses to the surroundings during the storage of liquefied, low-boiling gaseous mixtures, e.g., natural gas, in a storage vessel or a tanker, comprising withdrawing the liquefied gaseous mixture from the storage means, subcooling said liquefied mixture, and recycling resultant subcooled liquid into the storage means, the refrigeration value of the subcooled liquid being sufficient to compensate for the loss of refrigeration values due to heat from the surroundings. Savings in energy are obtained by virtue of the relatively high temperature of subcooling the liquid, as compared to the temperature required to condense vapor in equilibrium with the stored liquefied gas, and containing a substantial proportion of very low boiling components.

United States Patent Etzbach et al.

COMPENSATION OF REFRIGERATION LOSSES DURING THE STORAGE OF LIQUEFIED,LOW-BOILING GASEOUS MIXTURES Inventors: Volker Etzbach, Munich;Karlheinz Muller, Egling, both of Germany Linde Aktiengesellschaft,Wiesbaden, Germany Filed: Dec. 10, 1973 Appl. No.: 423,395

Assignee:

Foreign Application Priority Data Dec. ll, 1972- Germany 2260516References Cited UNITED STATES PATENTS 4/1974 Delahunty 62/54 PrimaryE.ran'zinerWilliam F. ODea Assistant ExaminerRonald C. CaposselaAttorney, Agent, or Firm-Milieu, Raptes & White [57] ABSTRACT A processfor the compensation of the refrigeration losses to the surroundingsduring the storage of liquefied, low-boiling gaseous mixtures, e. g.,natural gas, in a storage vessel or a tanker, comprising withdrawing theliquefied gaseous mixture from the storage means, subcooling saidliquefied mixture, and recycling resultant subcooled liquid into thestorage means, the refrigeration value of the subcooled liquid beingsufficient to compensate for the loss of refrigeration values due toheat from the surroundings. Savings in energy are obtained by virtue ofthe relatively high temperature of subcooling the liquid, as compared tothe temperature required to condense vapor in equilibrium with thestored liquefied gas, and containing a substantial proportionof very lowboiling components.

8 Claims, 1 Drawing Figure M] an 7,

US. Patent Nov. 11, 1975 3,918,265

COMPENSATION OF REFRIGERATION LOSSES DURING THE STORAGE OF LIQUEFIED,LOW-BOILING GASEOUS MIXTURES BACKGROUND OF THE INVENTION This inventionrelates to a process for the compensation of refrigeration losses duringthe storage of liquefied, low-boiling gaseous mixtures, and isespecially applicable to mixtures having widely spaced apart boilingpoints of the individual components.

For the compensation of evaporation losses during the transportation ofliquefied gases, a process is known for the recondensation of gasesevaporating from a plurality of storage tanks containing liquefied gases(DAS[German Published Application1No. 1,086,225). According to thisprocess, the evaporated gas from each of the storage tanks is liquifiedin a closed cycle by heat exchange with a liquid medium, the temperatureof which is below the boiling point but above the solidification pointof the liquefied gases. The liquified gas is recycled into the same tankfrom which it evaporated. To compensate for refrigeration losses ofstored liquid gas mixtures, this conventional process requires aconsiderable expenditure of energy because equilibrium considerationsdictate that the vapor phase above the liquid bath in the storage tanksis much more enriched in low-boiling components than the liquid itself.

SUMMARY OF THE INVENTION An object of this invention is to provide animproved energy-saving and stucturally simple process for thecompensation of the refrigeration losses during the storage oflow-boiling, liquefied gaseous mixtures in at least one storage tank.

Upon further study of the specification and appended claims, furtherobjects andadvantages of the invention will become apparent to thoseskilled in the art.

To attain these objectives, a system is provided comprising withdrawingliquefied gaseous mixtures from the storage tank, subcooling same inheat exchange with an auxiliary refrigerant, and recycling resultantsubcooled mixture into the storage tank.

By virtue of this invention, it is possible to provide makeuprefrigeration without condensing evaporated liquefied gaseous mixtures.Thus, it is unnecessary to condense at low temperature levels a vaporfraction highly enriched in the low-boiling components of the mixture.Thus, an important savings in energy is gained because the subcooling ofthe mixture present in the liquid phase can be conducted at aconsiderably smaller temperature difference (AT) between the subcooledliquid and the stored liquid than between the condensed vapor and thestored liquid. This concept of subcooling liquid instead of evaporatedvapor is generally unorthodox in the storage of liquefied gases becauseit is well-known that the overall heat transfer coefficient is largerand thus the size of heat exchanger is smaller when condensing vapors.Thus, while the present invention is contrary to conventionalengineering, energy is conserved.

The present invention is applicable to the storageof all liquefied gaseswhich would otherwise lose refrigeration value because of warming by thesurroundings. The invention is especially valuable in connection withliquefied gases having a boiling point less than 40 C preferably lessthan 150 C. Also, the invention is particularly important when treatinggaseous mixtures having about 95 to 0.1percent, preferably 15 to0.5percent of a component boiling lower than the boiling point of themixture.

The process of this invention is especially advantageous for thecompensation of the cold losses of stored liquid mixtures wherein theboiling points of the individual components are widely spaced apart,since in such mixtures the temperature difference between the boilingpoint of the liquid and the boiling point of the vapor phase inequilibrium with the liquid is particularly large. Thus, for example,the boiling point of a liquid natural gas composed of about 99 molarpercent of methane and 1 molar percent of nitrogen, stored under apressure of about 1 atmosphere absolute, is approximately 107 K. Thevapor phase in equilibrium with the liquid natural gas consists of aboutmolar percent of methane and about 20 molar percent of nitrogen having aboiling point at approximately 86 K. Accordingly, the present inventionis especially advantageous when the boiling point difference between twocomponents of the mixture are at least 10C, preferably at least 20C, andmost preferably at least 30C. Stated in another way, the invention isparticularly important when the difference between the boiling points ofthe stored liquefied gases and vapor in equilibrium therewith is atleast 1C, preferably at least 5C, and most preferably at least 20C.

A further advantage of the present process resides in that the coolingcurve of the liquid gaseous mixture can be adapted more efficiently tothe warming curve of the auxiliary refrigerant than is possible in caseof the corresponding curves of the vapor mixture and the auxiliaryrefrigerant. Therefore, the heat exchange itself required to compensatefor the refrigeration losses can be effected, according to the processof this invention, with lower temperature differences, which again isparticularly advantageous from an energy viewpoint. Thus there are twoenergy savings one from the temperature level of refrigeration which islower in the present invention, and the other is the lower temperaturedifference during the heat exchange with the auxiliary refrigerant.

When a liquefied nitrogen-containing natural gas is being stored, it isadvantageous for the auxiliary refrigerant to be nitrogen, which can becooled in a closed Claude refrigeration cycle, for example, or in anyother conventional refrigeration cycle.

In the event that the liquid gaseous mixture is stored in severalstorage tanks, for example in a tanker having a plurality of storagecompartments or storage vessels, a preferred embodiment of thisinvention comprises placing the vapor spaces of the individual storagemeans in communication with one another and to withdraw liquid gaseousmixture from only one container. This liquid is then subcooled in heatexchange with the auxiliary refrigerant, and then distributed among theindividual storage means so that the amounts of subcooled liquid mixturereturned into the storage containers are just sufficient to compensatefor the refrigeration losses caused in each instance.

The communication between the individual storage means is preferablyconducted via the vapor spaces in each storage means. The reason forthis is that a system consisting of several vessels can be considered asa single thermodynamic unit.

Because of the communication between the vapor spaces of the individualstorage means, some vapor is necessarily going to be transferred betweenstorage vessels so as to compensate for the loss of liquid from thestorage means used to deliver liquid for subcooling purposes.Accordingly, to avoid vapor from accumulating in the just-recitedstorage means, the distributed subcooled liquid is recycled in largeramounts into this storage tank, thus condensing such vapors.

The advantages of this preferred embodiment reside in that there isrequired only a single heat exchanger for subcooling the liquid gaseousmixture, and only a single pump to convey this mixture from the storagetank to the heat exchanger and from there to the individual storagetanks. Any enrichment in lower-boiling component in the liquid phase ofthe storage tank where the pump is arranged, which is possible duringfull load cooling, is not deleterious in most cases.

Moreover, a still further advantage is realized insofar as after thestorage tanks are emptied, a small amount of liquid need be retained ineach tank, ordinarily, in order to avoid warming of the tank because ofthe heat transfer from the surroundings. By virtue of the presentinvention, only in the one storage tank from which the liquid gaseousmixture is withdrawn for subcooling is it necessary to keep liquid,rather than, as heretofore customary, in each of the storage tanks.Thus, in the pres ent system, all other tanks can be completely emptied.This is of significant economic importance in tanker operations becauseafter the return voyage of the ship from the consumer (where the liquidgaseous mixture is unloaded) to the producer (where the ship isreloaded) a smaller quantity of liquid gaseous mixture can be left inthe storage tanks, so that, in total, the useful storage capacity of theship is better utilized.

In accordance with a further feature of this invention, it isadvantageous to conduct the heat exchange of the liquid gaseous mixturewith the auxiliary refrigerant in switching heat exchangers, so that anysolids which may freeze out during this heat exchange, e.g., during thestorage of liquid natural gas, higher-boiling hydrocarbons or solid COdo not result in a stoppage of the process since by conventionalswitching operations, a partially clogged heat exchanger path can beexchanged by a cleaned heat exchanger path.

DESCRIPTION OF PREFERRED ILLUSTRATED EMBODIMENT The drawing is aschematic view of a system comprising a plurality of individual storagemeans as found in tankers transporting liquefied nitrogencontainingnatural gas.

Referring to the drawing, there is a first storage tank 1 and a secondstorage tank 2. When the tanker travels from the producer to theconsumer, both tanks are filled almost completely with liquid naturalgas. A pump 3 is disposed within a lower zone of the first storage tank1, conveying liquid natural gas from this storage tank 1 through thevalve 4a, which is open during this switching step, and the crosssection 5a of the heat exchanger 6a. In the heat exchanger 6a, theliquid natural gas is subcooled in indirect heat exchange withlowtemperature nitrogen flowing in cross section 70 and derived from aClaude closed refrigeration cycle (not shown) and then recycled into thestorage tanks 2 and 1 via valves 10 and 11. The valve 10 is set so thatthe amount of subcooled natural gas flowing through this valve is justsufficient to compensate for the refrigeration losses caused by the heatfrom the surroundings Q In contrast thereto, the setting of valve 11makes it possible to convey a somewhat larger amount of sub coolednatural gas into the storage tank 1, so that there is the possibility,on the one hand, to compensate for the refrigeration loss due to Q and,on the other hand, to recondense the amount of natural gas flowing viaconduit 12 from the second storage tank 2 into the first storage tank 1and vaporized by Q To remove any solid deposits caused by frozenout COor solidifying higher-boiling hydrocarbons, a warm flushing step isemployed. Thus, there is provided a second heat exchanger 6b having acorresponding cross sections 5b and 7b, to which the process can beswitched over during the flushing of the first heat exchanger. Filters9a and 9b, connected downstream of the heat exchangers 6a and 6b,prevent any entrained solids from cross section 5a or 5b from enteringinto the storage tanks.

If the tanker is on its way back from the consumer to the producer,rather than as illustrated on its voyage from the producer to theconsumer, it is sufficient to compensate for the refrigeration lossesmerely by leaving a small amount of liquid natural gas in the firststorage tank 1, whereas the second storage tank 2 can be entirely empty.The amount of subcooled liquid natural gas conveyed through valve 10into the second storage tank is metered so that it just compensates forthe cold losses Q while completely evaporating. After this evaporation,the quantity of gas flows back into the first storage tank 1 and isthere recondensed by heat exchange with subcooled natural gas. Thislatter heat exchange is conducted by direct heat transfer relationship.

From the foregoing description, one skilled in the art can easilyascertain the exxential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:

l. A process for the compensation of the refrigeration losses to thesurroundings during the storage of liquefied, low boiling gaseousmixtures in a plurality of storage means, each having a vapor space anda lower zone for said liquefied gaseous mixture, the vapor spacesthereof being in communication with each other, said process comprisingwithdrawing said liquefied mixture from only one storage means,subcooling said liquefied mixture, and recycling subcooled liquefiedmixture into all of the plurality of storage means, with the provisionthat a larger portion of subcooled gaseous mixture is recycled into thestorage means from which the liquefied gaseous mixture is withdrawn,then into each of the remaining storage means.

2. A process as defined by claim 1, wherein the boiling point of thestored liquefied gaseous mixture is less than 40C.

3. A process as defined by claim 1, wherein the stored mixture isnatural gas containing a minor amount of nitrogen.

4. A process as defined by claim I, wherein said plurality of storagemeans comprise compartments of a liquefied natural gas tanker.

ference between the boiling point of the stored liquefied gaseousmixture and vapor in equilibrium therewith is at least 1C.

8. A process as defined by claim 1, wherein the subcooling is conductedby indirect heat exchange Contact with an auxiliary refrigerant.

1. A process for the compensation of the refrigeration losses to thesurroundings during the storage of liquefied, low boiling gaseousmixtures in a plurality of storage means, each having a vapor space anda lower zone for said liquefied gaseous mixture, the vapor spacesthereof being in communication with each other, said process comprisingwithdrawing said liquefied mixture from only one storage means,subcooling said liquefied mixture, and recycling subcooled liquefiedmixture into all of the plurality of storage means, with the provisionthat a larger portion of subcooled gaseous mixture is recycled into thestorage means from which the liquefied gaseous mixture is withdrawn,then into each of the remaining storage means.
 2. A process as definedby claim 1, wherein the boiling point of the stored liquefied gaseousmixture is less than 40* C.
 3. A process as defined by claim 1, whereinthe stored mixture is natural gas containing a minor amount of nitrogen.4. A process as defined by claim 1, wherein said plurality of storagemeans comprise compartments of a liquefied natural gas tanker.
 5. Aprocess as defined by claim 4, wherein the stored mixture is natural gascontaining a minor amount of nitrogen.
 6. A process as defined by claim1, wherein the components of the stored gaseous mixture have adifference in boiling point temperatures of at least 10* C.
 7. A processas defined by claim 1, wherein the difference between the boiling pointof the stored liquefied gaseous mixture and vapor in equilibriumtherewith is at least 1* C.
 8. A process as defined by claim 1, whereinthe subcooling is conducted by indirect heat exchange contact with anauxiliary refrigerant.